The invention relates to surgical products, and in particular, to surgical reamers for cutting shaped cavities in bone.
In order to produce a shaped cavity in bone for a hip implant, which requires smooth walls and accurate shape, it is advantageous that the reamer shell or cutting bowl be hemispherical. Further, the cutting teeth must be properly located and oriented. Still further, the tooth height is important to the size of bone chip and thus to the accuracy of the shape cut by the reamer.
In most cases, an implant in a hip socket is best fixed to a concave, hemispherical cavity. However, such a shape is not strictly necessary. Other acetabular cutting shells are non-hemispherical and so the principle explained here may be adapted to include such other geometries.
It is increasingly important, especially with cementless hip surgery, that the acetabulum be reamed to an exact form, generally a hemisphere, thus allowing maximal contact between the bone and the definitive (hemispherical) implant.
Further, there is increasing emphasis on cutting a smaller incision to minimize the trauma to the patient and to augment the rate of recovery. Meeting this additional requirement provides an additional challenge to the designers of medical instruments and implants. In addition, the change in surgical procedure includes the fact that the surgeon now more often maintains the acetabular reamer handle on a single axis rather than performing the step of “sweeping” the end of the tool handle through an angle and thus continuously changing the axis of the reamer cut. If a test is made maintaining a prior art reamer handle on a constant axis, then a series of concentric rings are cut that, on a macro-scale, approximate a hemisphere. When the surgeon “sweeps” the axis of the reamer handle, these irregularities are removed (in a similar manner to polishing) yielding a hemispherical surface.
In an effort to maximize the number of concentric rings, to minimize chatter/vibration and thus approach a smooth hemispherical surface without sweeping, it is desirable to add more teeth. However, when this is done, mechanical strength decreases. Further, it is more difficult in this case to insure that the cut profile of each tooth overlaps. Larger teeth have been attempted but either the chip size and cutting stresses were too large or the reamer was too complex. Further, due to the large opening adjacent the larger teeth, mechanical strength was sacrificed.
U.S. Pat. No. 5,116,165 to Sayler describes a reamer having a limited number of discrete blade-like teeth. These teeth are defined by a single curve of the profile of the form to be cut. In other words, these teeth are flat. Such a tooth form thus is not supported in that no structure is provided to help maintain the form of the tooth (other than the tooth itself) when faced with the sometimes unusually high cutting pressures of reaming. Further, the integrity of the spherical form of the reamer can be affected when there are a limited number of extensive slits of cuts in the spherical. This integrity is affected by the fact that high stresses are induced at the sharp corners of the slits.
It is often the case that the form of the tooth is a function of the original material form: the sheet material or the base diameter of the hemisphere or of the manufacturing method. Often no consideration is given to the form of the cut surface. Therefore, the cut of a single tooth often only approximates the required form of a sphere or a hemisphere. For example, it may yield a planar surface or have a radius different than that required and further generate an overall hemisphere that is irregular.
Therefore, what is needed is a reamer that minimizes the discrete cut surfaces and generate a series of cuts that comprise a single defined geometry. Further, what is needed is a tooth form that can be controlled independently of the form of the original material form.